Normally, turn off the primary earbud, waiting for 1-2s, and the secondary earbud will automatically connect the device.
If it can't connect, you can manually click the bluetooth name of the secondary earbud in the mobile bluetooth list.

 If this doesn't work, then reset the headphones refer to the manual and re-pair:

(1) Place the earbuds into the charging case and make sure they are being charged for at least 10 seconds.
(2) Keep the charging case open and press and hold the button on the charging case for 10s until the LED indicators flash red 3 times.
(3.)Then wait for 4S and the LED comes into white light flash quickly. You could connect your iPhone this time.